The present invention relates generally to the rheology and storage stability of coal derived materials. More particularly, it is concerned with a process for stabilizing the viscosity characteristics of coal derived fluid and fluid blend materials and with the stabilized materials obtained thereby.
The viscosity instability of certain coal derived fluid materials has caused substantial problems in the handling, storage, and utilization of these materials. The instability can result in a viscosity increase to levels greater than 10,000 cP in a matter of days thereby preventing the pumping and use of the material as a boiler fuel. Heretofore this instability has been attributed to oxidative aging; however, known oxidation inhibitors for petroleum-derived materials are ineffective to stabilize the viscosity of the coal derived fluid materials.
The materials of primary concern are the materials referred to as the residual oils of the solvent refined coal (SRC) process. These SRC residual oils are homogenous single-phase blends of SRC distillate liquids having a boiling range of 200.degree.-455.degree. C. and deashed SRC products having a boiling range in excess of 455.degree. C. derived from the first and second stages of the SRC process. The residual oil blends may be solid at ambient temperature and typically are heated and stored at elevated temperatures of about 65.degree.-120.degree. C. where, in the liquid state, they exhibit homogeneity and Newtonian behavior that permits their use as a No. 6 fuel oil substitute in conventional fuel handling equipment. For such use, the viscosity characteristics of the residual oils must be such as to facilitate pumping and proper atomizing of the fuel. As mentioned, it has been extensively reported that coal derived liquids of this type are very susceptible to oxidative degradation or aging that dramatically increases their viscosity thereby adversely affecting the pumping and atomizing temperatures of the residual oils. It is therefore essential that the viscosity characteristics of the materials be maintained within acceptable limits during storage for periods of four months and longer.
In accordance with the present invention, it has been found that SRC residual oils tend to lose their volatile low boiling constituents during blending, handling, and storage and that such loss results in an irreversible increase in viscosity that far exceeds any increase that might be expected simply from concentration changes. It is believed that these volatile constituents loosely associate with nitrogen containing molecules in the oils to control the viscosity during prolonged storage. Loss of these constituents tends to free the nitrogen containing materials to associate with heavier acidic molecules thus forming high molecular weight species of increased viscosity. The reaction forming these high molcular weight species is not reversible and therefore, it is believed, the petroleum-derived inhibitors are ineffective in stabilizing the viscosity of the residual oils after loss of the light volatile components.
The present invention provides a process for maintaining a stabilized viscosity by minimizing the loss of the light volatile components of the residual oils so as to prevent the high molecular weight acid/base aggregation and the resultant irreversible increase in viscosity associated therewith.
Those attributing the viscosity instability to oxidative aging have suggested blanketing the residual oils with an inert atmosphere such as nitrogen. While partially effective, even this technique does not prevent the loss of some of the necessary volatile components during the transfer of the residual oils between various storage facilities. As can be appreciated, even when extreme care is taken during transfer, the vapor of the volatile components will equilibrate within the vapor space in the new storage vessel. In accordance with the present invention, even this slight loss can be minimized, if not essentially eliminated.
In accordance with the present invention, it has been found that the suppression of the volatilization of low boiling components within the residual oil will be effective to maintain the viscosity of the oil at its desired low value without significantly diluting the oil. A benefit of this procedure is the increase in the amount of SRC used in the blend. With this new technique it can be increased above the 35 percent concentration used heretofore while extending the storage times well beyond that experienced for such materials. The volatile components typically are present in rather low concentrations such that viscosity stability of the blend can be maintained for extended periods of time through the use of equally low concentrations of a viscosity repressor. This process is designed for high efficiency of operation while minimizing loss of the volatile constituents and therefore minimize viscosity increases.
Other advantages and features will be in part obvious and in part pointed out more in detail hereinafter.
These and related advantages may be achieved in accordance with the present invention by providing a proces for stabilizing the viscosity of homogenous coal derived fluid and fluid blend materials during handling and storage at temperatures of as low as 35.degree. C. and as high as 150.degree. C. for a period of time up to about four months by blending a coal derived material, adding to the coal derived material a volatile phenolic viscosity repressor, feeding the coal derived material to a storage container while minimizing the loss of volatile components therein and maintaining a partial pressure of said phenolic viscosity repressor over said coal derived material during storage.
A better understanding of the invention will be obtained from the following detailed description of the process including the several steps and the relation of one or more of such steps with respect to each of the others and the product obtained therefrom possessing the features, characteristics, compositions, properties, and relation of elements described and exemplified herein.